1. Field of the Invention
This invention pertains to the field of lasers. More particularly, it pertains to a method and apparatus to perform the method of using electrical energy-generated plasma in a chemical oxygen-iodine laser (known as a xe2x80x9cCOILxe2x80x9d) to reduce the loss of energy generally encountered in the dissociation process of I2 that inherently robs energy from interaction with oxygen in the singlet delta state.
2. Description of the Prior Art
Lasers (an acronym for Light Amplification by Stimulated Emission of Radiation) are devices producing intense, coherent and typically highly directional beams of light. Lasers are based upon the fundamental process of stimulated emission, a process by which a collection of atoms, molecules, ions or other medium actually amplifies light energy. This amplification occurs due to an interaction in which the light field interacting with the excited atoms induces them to radiate light precisely in phase with the optical input signal being amplified. The condition under which light amplification can occur is referred to as a xe2x80x9cpopulation inversionxe2x80x9d. This description broadly applies to a condition in which the excited population density exceeds that of a lower level. This population inversion can be created through numerous pumping processes, chemical, optical, electrical, and others. An inverted population amplifies spontaneous emission leading to the effects of stimulated emission, light amplification and oscillation, sort of chain or domino effect, by which one atom""s emission of light quanta (photon) induces subsequent atoms to emit at the same frequency, in phase with and contributing to the initial optical emission. This process is the basic process of stimulated emission and is in general necessary to all laser systems.
Lasers (as an engineering device), in general, consist of three major sub-systems, (1)the light amplifying medium (gain medium), (2) the energy/power supply or xe2x80x9cpump sourcexe2x80x9d, xe2x80x9cpumping processxe2x80x9d or laser pump mechanism, that introduces energy into the gain medium and allows for light amplification, and (3) the optical resonator, or optical cavity laser that allows for laser oscillation to build up and a coherent optical laser beam to be extracted for various applications. Lasers have multiple variations on the type of gain media, the energy source and the optical resonator system.
All lasers require some form of energy to pump or excite the gain medium and create the population inversion needed for laser action (called xe2x80x9clasingxe2x80x9d). Chemical lasers directly utilize energy released by a gas phase chemical reaction to create the atomic or molecular population inversion needed for a laser device.
Atomic iodine lasers were demonstrated relatively early in the era of laser technology in about 1964. The earlier atomic iodine lasers were characterized by near infrared emission and generally only in brief pulses. These lasers were usually pumped through an optical process involving intense flash-lamps to dissociate an iodine compound and lead to the excited iodine. In general, these lasers were pulsed and were of interest for research involving short duration, very powerful optical pulses for studying laser/matter interactions at high intensity. Beginning in the late 1970""s the consideration was given for chemical reaction schemes for producing the excited atomic iodine laser state for high average power, military class lasers (high energy laser xe2x80x9cHELxe2x80x9d weapons).
A particular chemical laser is the chemical oxygen iodine laser or simply the xe2x80x9coxygen-iodinexe2x80x9d laser. This system is complex, and requires some explanation of the overall process. Chemical energy is utilized in a complex reaction between an excited state of molecular oxygen and atomic iodine. The excited state of molecular oxygen is called a xe2x80x9csinglet delta statexe2x80x9d of oxygen and is designated in molecular spectroscopic terms as xe2x80x9cO2(a1xcex94)xe2x80x9d. O2(a1xcex94) may be produced by a wide variety of reactions. A common production reaction is that between basic hydrogen peroxide (hydrogen peroxide with an alkaline hydroxide, such as potassium hydroxide, added) and chlorine gas. There are other known methods of producing the excited state of molecular oxygen O2(a1xcex94) that include chemical, electrical and optical means and hybrids of them. This invention does not require any specific means of producing singlet delta state oxygen.
There are other gasses that may be combined with atomic iodine in a gas laser. One such gas is the excited state of molecular nitrogen chloride, known as xe2x80x9csinglet nitrogen chloridexe2x80x9d and is designated in molecular spectroscopic terms as xe2x80x9cNCl(a1xcex94)xe2x80x9d. This singlet nitrogen chloride is also useful in the instant invention.
The optimal energy transfer partner atom is atomic iodine. This excited state is not, by itself, suited to laser operation. However, it can be mixed with another gas, and through collisions transfer it""s excitation energy to an atom suited for laser action. Such a scheme is called xe2x80x9ccollisional resonant energy transferxe2x80x9d. Such lasers use an excited energy xe2x80x9cdonorxe2x80x9d species to transfer energy (through molecular to atomic collision) to a receptor species (the actual lasing species).
The chemical oxygen-iodine laser (known as a xe2x80x9cCOILxe2x80x9d) was first demonstrated as a chemical laser system in 1978-79. The laser showed promise for scaling to high average powers. However, a problem with the COIL system was recognized at this early development phase.
The iodine atom must first be generated by the dissociation of molecular iodine vapor. This is done in conventional COIL systems by injecting I2 vapor into a flow of O2(a1xcex94). In a complex and not fully understood process, anywhere from four to seven separate O2(a1xcex94) excited molecules sequentially collide with the I2, each depositing excitation energy before the I2 chemical bond is finally overcome, and free iodine produced. This process, while providing atomic iodine needed for lasing, also very significantly depletes the energy stored in the O2(a1xcex94) excited state flow. Energy depleted from the O2(a1xcex94) population and flow field results in a loss in laser power. Estimates are that up to 15% of the chemical energy potentially available for high power laser output is lost by this inefficient iodine dissociation step to generate free iodine atoms. The remaining portion of un-depleted O2(a1xcex94) is then used to actually pump iodine atoms on the 1.315 micron, near infrared, laser transition. A micron=1 micrometer or 1 millionth of a meter and is a standard unit of optical wavelength).
The conventional COIL system is typically configured as a supersonic flow laser. Subsonic flow versions of COIL devices have also been demonstrated. In this configuration, helium, or other inert and O2(a1xcex94) are expanded from a higher pressure xe2x80x9cplenumxe2x80x9d through a supersonic nozzle. Molecular iodine vapor is injected at or near the nozzle throat and the conventional mixing/dissociation process occurs. Subsonic flow versions of COIL devices have also been demonstrated. In conventional existing COIL systems the design uses a helium carrier gas for the chemically generated excited O2(a1xcex94) state. This mixture of helium and excited oxygen is transported to the nozzle or throat of a supersonic expansion nozzle. Molecular iodine is injected into the expanding O2(a1xcex94)+ helium flow at or near the nozzle throat. This mixing results in a complex multi-step reaction by which four to seven separate O2(a1xcex94) molecules are required to break up one iodine molecule of I2. This robs the COIL system of useful O2(a1xcex94) to drive the laser process with iodine atoms. As a result the energy and power output of a COIL device can be decreased by a substantial amount (about 15%) compared with what should be possible, were all of the O2(a1xcex94) made available for powering iodine atom laser action.
Further, the mixing and injection of the molecular iodine into the expanding O2(a1xcex94) and helium gas flow results in complex gas density variation patterns that extend into the optical laser region of the flow. This injection process is a result of the need to mix the iodine into the expanding O2(a1xcex94) flow in the nozzle at the last possible stage prior to gas flow into the optical cavity or lasing zone. Any attempt to homogenize the gas mixture by pre-mixing the iodine and excited oxygen O2(a1xcex94) triggers the dissociation mechanism and oxygen energy depletion. Traces of water vapor from the O2(a1xcex94) chemical formation process act as deactivators for the excited states. This deactivation rate is proportional to the square of the local pressure. Thus, mixing oxygen in the O2(a1xcex94) state with iodine at higher pressures, such as 500 torr up to atmospheric pressure, results in large energy losses for laser action. A xe2x80x9ctorrxe2x80x9d is a standard unit of pressure equal to 1,333.22 microbars or the pressure needed to support a column of mercury one millimeter high under standard conditions. Thus, due to I2 energy reactions and dissociation processes with O2(a1xcex94), along with attendant high pressure dependent water vapor and iodine deactivation losses, the present COIL technology must rely on injection of I2 in the nozzle throat to allow for time to dissociate the I2 while attempting to minimize the O2(a1xcex94) depletion losses. This regime of operation results in attempting to balance contradictory conditions to avoid additional losses to laser performance.
In summary, the problems presently existing in prior art conventional COIL lasers include depletion of the O2(a1xcex94) energy donor molecules by the I2xe2x86x922I atoms dissociation process resulting in an overall loss to the extraction of chemical energy as laser light output and further efficiency loss in laser performance by the inability to mix all O2(a1xcex94) excited energy donors with I-atoms (incomplete mixing) in the complex supersonic flow field of COIL technology.
This invention is a method, and an apparatus for performing the method, for significantly reducing and, in some cases, eliminating, the above-described problems with COIL lasers. The process includes the steps of obtaining a quantity of oxygen in the singlet delta energy state [O2(a1xcex94)], mixing O2(a1xcex94) with a quantity of non-reactive carrier gas to produce a homogeneous mixture, combining this mixture with a non-reactive iodine donor gas just prior to passing the entire combination through a supersonic nozzle, creating an electrical discharge plasma downstream from the supersonic nozzle, passing the combination of gasses through the supersonic nozzle and the discharge plasma at high mass flows and velocities to dissociate the iodine into free iodine atoms and direct the flow of gasses and free iodine atoms from the discharge plasma into the optical cavity laser action region of the laser.
The suitable non-reactive iodine donor compound is preferably mixed with the O2(a1xcex94) and the helium in a plenum chamber in order to obtain homogeniety of the combination. The non-reactive iodine donor compound is chosen that has no reactions with the energetic O2(a1xcex94). The resultant mixture is now homogeneous and expands through the nozzle to support the high mass flow of the gasses as in a conventional supersonic flow chemical iodine laser. The discharge plasma is carefully selected to maximize iodine atom yield by optimized plasma dissociation of the iodine compound. The gas mixture exits the nozzle, now with free iodine atoms generated by flowing through the RF plasma zone, for entrance into the optical cavity laser action region. The plasma is achieved by locating at least a pair of spaced-apart RF (radio frequency) or DC (direct current) electrodes within the nozzle or at the nozzle exit and energizing them with electrical (radio frequency or direct current) energy.
Accordingly, the main object of this invention is a process for increasing the power of a COIL by dissociating the iodine molecule with radio frequency energy thus preventing loss of energy from the excited oxygen stream. Other objects and benefits of the invention include removal of gain medium optical quality distortions created by prior art methods of injecting and mixing heavy I2 into the energetic O2(a1xcex94) flow that results in disruptions to the gain medium optical density and further results in beam quality distortions that lead to a decreased intensity and focus capability of the output beam; a method for reducing the loss of excited oxygen O2(a1xcex94) during the dissociation reaction of I2xe2x86x922I atoms; a method of achieving more homogeneity in the mixture of O2(a1xcex94), I2 and carrier gas; and, a method of relieving the attenuation of the laser reaction by the presence of water vapor.
These and other objects of the invention will become more clear when one reads the following specification, taken together with the drawings that are attached hereto. The scope of protection sought by the inventors may be gleaned from a fair reading of the Claims that conclude this specification.